The "something falling off the shelf" is Janie's illusion/perception of Joe Starks. Up until this point, Janie has held on to her hopeful beliefs that Jody wanted to be a "big voice" and was the man for her. She kept this illusion alive, even after the little signs that she had made a mistake. Jody put her down, forced her to work in the store, and cover up her beautiful hair. But through it all, Janie waited for the man she believed he was in their initial courtship to resurface.
But, after Jody slaps Janie for burning dinner, she lets all her illusions of Jody fall away. Her comment of going to "see what it was" is her own realization that she had been kidding herself about the kind of man her husband was.